Github user aarondav commented on the pull request:
https://github.com/apache/spark/pull/182#issuecomment-38132716
This _is_ an incompatible change, which is why it is targeted for 1.0. It
is my opinion that this is how it should've been in the first place; since
Spark is inherently a distributed system, it doesn't make sense for an
out-of-the-box "spark-shell" to run on one thread.
We use availableProcessors() elsewhere in the Spark codebase, and here it
is used only as a reasonable default. "local" mode is only intended for testing
purposes, so the impact and severity of this change is limited.
(Also I'd like to point out that the tests that needed to be modified were
actually making false assumptions about partition ordering that just happened
to work out since it was using 1 thread.)
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